The researchers looked at data from late November to early February from 3,144 children and adults, 1,650 of whom were vaccinated, to see who sought medical treatment for flu-like symptoms.

While the vaccine was found to be 48 percent effective in preventing flu-related medical visits for all ages, it provided slightly better protection for children 6 months to 8 years old and for adults age 50 to 64, according to the report. The vaccine was found to be 53 percent effective in preventing flu-related medical visits for the young children and 58 percent effective for the older adults.

Meanwhile, it was found to be less effective among those age 9 to 17 (32 percent effective), 18 to 49 (19 percent effective) and 65 and up (46 percent effective).

"We know that influenza vaccine is a good but not perfect vaccine," said Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center.

Schaffner pointed out that it is especially important that the elderly and those who are around the elderly get the vaccine, since the dominant flu strain A(H3N2) spreading across the country is more likely to cause severe complications among the elderly.

The current flu vaccine has been found to protect against the A(H3N2) strain 43 percent of the time, and it can lessen the chances of an infected person's developing serious symptoms, according to the MMWR.

"It disproportionately affects older people and makes them sicker," Schaffner said of the A(H3N2) strain. "There is a perfect match between that strain and what is in the vaccine."

The flu vaccine is developed every year to match the virus strains that are expected to be most common during flu season in the U.S. The country is in the middle of a flu epidemic, which occurs almost every year. The CDC report found that a large numbers of flu cases are likely for the next few weeks.

Flu symptoms can include headache, fever, joint pain and cough. The flu season usually runs from November through March in the U.S., with the number of cases often peaking in February. The number of people affected each year can vary widely, but generally, the CDC reports that "millions of people are sickened, hundreds of thousands are hospitalized, and thousands or tens of thousands of people die from flu every year."